1. Field of the Invention
The present invention relates to a vapor drier used to dry precision component parts in electronic, optical and other fields, and more particularly to a vapor drier in which a vapor drying liquid can be regenerated and reused and in which the amount of water contained in the vapor drying liquid can be controlled to a fixed level or below.
2. Description of the Related Art
As a method of drying an object to be dried after washing, a so-called vapor drying method is known. This drying method is carried out by using a drying liquid consisting essentially of an organic solvent which is very hydrophilic and which has a low boiling point, such as isopropyl alcohol (hereafter also referred to as IPA).
Specifically, this drying method can be implemented by using an apparatus whose basic construction is such that a cooling section is provided in its upper portion, and a vapor generating tank which can be heated is disposed in its lower portion. If an object to be dried is placed in an open space above the vapor generating tank, IPA vapor evaporating from the heated vapor generating tank is condensed on the surface of the object to be dried. Since the condensed IPA flows down together with the water adhering to the object to be dried, the drying of the object to be dried is accomplished.
If IPA condensed on the surface of the object to be dried is allowed to drop into a drying liquid tank, the concentration of water in IPA soon becomes high, and the concentration of water in the evaporating IPA vapor also becomes high. Consequently, the drying efficiency declines.
Accordingly, it is conceivable to adopt a structure in which all the IPA condensed on the surface of the object to be dried is discarded to prevent the condensed IPA from entering the vapor generating tank. If this structure is adopted, it is possible to prevent water from being directly mixed with IPA in the tank.
In the apparatus, however, the IPA vapor which did not condense on the surface of the object to be dried is refluxed in the cooling section located in an upper portion of the apparatus, is returned to the drying liquid tank, is heated again, and is evaporated on a repeated basis.
Meanwhile, even if the above-described structure, in which IPA condensed on the surface of the object to be dried is prevented from directly entering the vapor generating tank, is adopted, water is directly evaporated, although in small amounts, from water droplets attached to the object to be dried. The water vapor is refluxed together with the IPA vapor.
Accordingly, as dry processing is repeated, the concentration of water in the drying liquid in the tank increases, so that the above-described problem occurs.
It is known that the above-described problem is particularly important in cases where the objects to be dried are semiconductor wafers.
It should be noted that a description of driers is given on pages 68-71 of the March 1983 issue of electronic Parts and Materials published by Kogyo Chosakai Publishing Co., Ltd.
In addition, a technique whereby the concentration of water vapor in the drying liquid vapor in the vapor drier can be automatically controlled to a low level is disclosed in Japanese Patent Application Laid-Open No. 45127/1987.
In this technique, in a vapor drier having discarding means for discarding a drying liquid in a vapor generating tank and supplying means for supplying new drying liquid, when the concentration of water in the drying liquid in the vapor generating tank reaches a predetermined value, all or part of the drying liquid in the tank is discarded. New drying liquid is replenished, thereby making it possible to control the concentration of water in the drying liquid in the vapor generating tank to a predetermined level or below. At the same time, the content of water vapor in the drying liquid vapor evaporating from the vapor generating tank is controlled to a predetermined level or below.
By virtue of this technique, the concentration of water in the drying liquid can be controlled, but the continual replenishment of the new IPA according to the above-described method results in a rise in cost. The problem of disposing of the used IPA also arises.
Therefore, in order to obtain a distillate whose water concentration is held down to not more than a level which does not present a problem in the drying of precision electronic and optical components by using a known distillation method, a distillation device becomes complex and large in scale. In the case of IPA, for instance, since an IPA concentration in the vicinity of 88 wt.% exhibits an azeotropic composition with respect to water, it is impossible to obtain an IPA having a higher concentration than the aforementioned level by means of a normal distilling operation. In this case, as a generally used concentrating method, a method is known in which azeotropic distillation is effected by adding a benzene entrainer. This method requires at least three towers, i.e., a dehydrating tower using the entrainer, a tower for removing water collected by the entrainer, and an IPA refining tower. In order to obtain a distillate in which the water content is held down to not more than a level which does not present a problem in the drying of precision electronic and optical components, a distilling tower normally becomes 6 m or higher, and hence occupies a large space in a clean room of a plant for manufacturing precision electronic or optical components. If such a complicated distilling operation is conducted, the cost of equipment becomes high, and the adoption of this method is quite difficult in terms of space. In addition, it has been difficult to carry out the regeneration of IPA in terms of energy cost as well.
In contrast, a technique which employs a membrane separation operation instead to the distilling operation in a dehydration process is disclosed in Japanese Patent Application Laid-Open No. 239628/1986. The following steps are adopted in this apparatus: cleaning semiconductors by means of the vapor of an organic solvent; processing and dehydrating waste liquid of the organic solvent by a pervaporation method; distilling the dehydrated organic solvent; and circulating the organic solvent. In this case, since most of the dehydrating operation is effected by the membrane separation operation, a distilling device can be made compact, but a space equivalent to or larger than that of a vapor drier body is still required for the dehydrating step. In addition, since a membrane type separator and the distilling device are operated separately, this technique has been disadvantageous in terms of the energy cost.
Particularly in recent years, in conjunction with the trend toward greater integration and higher precision of semiconductors and liquid crystals, there has been a demand for maintaining the concentration of water in the vapor drying liquid at a low level and for processing a large amount of drying liquid in a small space so as to improve productivity. For instance, in order to effect dry processing of fifty 6-inch wafers/batch for 16 M DRAMs, it is required to set the water concentration in IPA to 0.5% (5000 ppm) or less and the rate of processing with IPA to 10 kg/hr.